Conductive polymers are used in a number of applications including fibers, films, and coatings. Acid-doped polyaniline is often a focus of developmental work with conductive polymers, and solutions containing polyaniline in a solvent are commercially available.
One of the most important uses for conductive polymers is as the electrolyte layer in capacitors made from pressed powders. In such articles, one or more coatings of electrically conductive polymer can be used as a solid electrolyte between the dielectric oxide layer and the electrodes. With some systems, the porous, anodized substrate is impregnated with a solution containing the monomer or monomers. A polymerization agent is added (if not mixed with the monomers), and the conditions are adjusted to polymerize the monomers. The polymer is then washed, and the anode is "reformed" to activate the anode. Several impregnation and reformation steps may be used.
One polymerization agent of interest is a ferric (Fe.sup.3+) salt as an oxidizing agent at a 1:1 molar ratio with the monomer to polymerize pyrrole or ethylenedioxythiophene (EDT) monomers. The polymerization can be performed in organic or aqueous systems, and the resulting films or powders are highly conducting. See, U.S. Pat. Nos. 4,697,000 and 4,847,115 (combination of strong oxidant and dopant anion for pyrrole polymerization) and U.S. Pat. No. 5,035,926 (iron (III) salt for polymerization of EDT).
Unfortunately, the reaction rate of the polymerization is very difficult to control. It is also difficult to remove all of the resulting ferrous (Fe.sup.2+) salt produced by the reaction, particularly when the oxidation polymerization is in-situ on a porous body. Residual iron in a capacitive body may cause high leakage current rates, ionic migration in humid environments, or both.
Previous attempts to replace the ferric salt with another oxidizing agent have not been able to produce a commercially viable system. Ammonium persulfate will perform the oxidation adequately but is even more difficult to control than ferric salt. The use of molecular oxygen to regenerate the ferrous salts has degraded the polymer and posed problems in controlling the activity level of the oxygen. See U.S. Pat. No. 4,604,427 that discloses the use of various oxidation agents to polymerize cyclic monomers into conductive polymers.
It would be useful to have a reliable system for oxidizing monomers to form a conductive polymer without leaving unacceptable levels of undesired conductive salts.